The present invention relates to a cutoffless class B emitter-follower type SEPP (Single Ended Push-Pull) circuit.
In general, an emitter-follower type SEPP circuit is operated in class B for reason of efficiency. In order to provide a smooth transition between upper and lower transfer characteristics, the establishment of idle current flows is essential. In such an ordinary circuit, when one of the transistors is turned on, the other transistor is cut off, as a result of which switching distortion occurs. In order to overcome this difficulty, a cutoffless class B circuit is generally employed in which, with the aid of a servo circuit, neither of the transistors is cut off to cause certain amounts of idle current to flow at all times. In this case, it is true that the switching distortion is eliminated. However, there is still present current distortion due to the nonlinear, exponential current transfer characteristics of the transistors, and also a voltage distortion attributed thereto.
Furthermore, in the case of bipolar transistors, the presence of idle currents may lead to thermal runaway in the absence of temperature compensation. Yet further, the idle current value tends to vary according to the presence or absence of a signal or the ambient temperature. Thus, the operating point changes over both long and short periods of time irrespective of the presence or absence of the signal.
The aforementioned temperature compensation is extremely critical. Therefore, it is considerably difficult to design a circuit having adequate temperature compensation. Especially in the conventional cutoffless class B circuit, the idle currents have low stability because of the presence of negative feedback. This, together with the fact that it is difficult to perform complete temperature compensation because of the nature of the circuit, increases the difficulty in design.
FIG. 1 shows the fundamental arrangement of a conventional cutoffless class B SEPP circuit. In FIG. 1, A.sub.1 and A.sub.2 designate error amplifiers whose gain is unity or less; B.sub.1 and B.sub.2, voltage generating circuits, or voltage adders; C, an input signal source; and V.sub.B, bias sources for transistors Q.sub.1 and Q.sub.2.
In FIG. 1, the currents i.sub.E1 and i.sub.E2 flowing during "silent" periods in which no signal is applied to the input terminal IN are the idle currents I.sub.d. Currents I.sub.B and I.sub.B2 are supplied by the power sources V.sub.B. Each idle current I.sub.d has a level: ##EQU1## ps where V.sub.BE is the base-emitter voltage of the transistor and R.sub.E is the emitter resistance. When an input signal current i.sub.i flows, the current i.sub.E1 is increased to: EQU i.sub.E1 =h.sub.fe1 .multidot.i.sub.i,
where h.sub.fe1 is the current amplification factor of the transistor Q.sub.1.
The input voltage Vi.sub.1 to the amplifier A.sub.1 is: ##EQU2## If the amplifier A.sub.1 were not provided, this voltage would cut off the transistor Q.sub.2 by reversely biasing the base. However, due to the presence of the amplifier A.sub.1 having a gain of unity, the voltage vi.sub.1 is positively fed back to the base of the transistor Q.sub.1 to raise its potential, and therefore a certain amount of idle current I.sub.d always flows without reversely biasing the transistor Q.sub.2. In the case also where the input signal current i.sub.i is inverted in polarity to turn on the transistor Q.sub.2, the same operation is carried out and the transistor Q.sub.1 is not cut off. This is the operation of a cutoffless class B SEPP circuit.
FIG. 2 shows a current transfer characteristic with respect to the input signal current in the circuit of FIG. 1. In general, when the emitter current of a transistor increases, the current amplification factor h.sub.fe1 decreases abruptly, and accordingly the combined characteristic curve is considerably nonlinear as indicated in FIG. 2, and hence a large current distortion is caused. If the gains of the amplifiers A.sub.1 and A.sub.2 are unity as described above, then the positive feedback percentage is 100%, and therefore the action of stabilizing the idle current I.sub.d due to the presence of the resistor R.sub.E is completely lost. This is equivalent to the fact that R.sub.E becomes zero in the above-described expression (1). Accordingly, the idle current becomes unstable, thus resulting in an oscillation. In practice, the gains of the amplifiers A.sub.1 and A.sub.2 are set to less than unity; however, the idle current is still unstable with temperature.
If to eliminate the distortion caused by the current transfer characteristic, a constant voltage drive circuit is formed by removing the amplifiers A.sub.1 and A.sub.2 from the cutoffless class B SEPP circuit and employing a constant voltage source as the input signal source C, the transfer characteristic is as shown in FIG. 3. Even in this case, the distortion attributed to the exponential transfer characteristic of the transistors remains unchanged.
In the conventional class B SEPP circuit employing the constant voltage drive method, (1) distortion arising due to the exponential function transfer characteristic and switching distortion attributed to the on-off operation of the output transistor are produced. Even in the cutoffless class B SEPP circuit, (2) distortion due to the current transfer characteristic occurs as described above. In addition, in any drive method, (3) temperature compensation is required for the idle current, and even when this is provided, it is impossible to completely compensate the idle current. (4) It takes a long period of time (more than thirty minutes or so) for the idle current to become constant after the power switch has been turned on. (5) The idle current varies depending on whether or not an input signal is present, and the magnitude of the idle current is greatly shifted from the set value when a large signal has been inputted. (6) Because of the above-described drawbacks (3) through (5), the operating point is unstable, varying with the ambient temperature and the presence or absence of the input signal.